1. Field of the Invention
The invention relates to an eddy current sensor and an eddy current measurement method and, more particularly, to a technique for improving the inspection accuracy of a hardness penetration using eddy current measurement.
2. Description of Related Art
For example, a steel product (hereinafter referred to as steel product) subjected to induction quenching is used for mechanical components, such as engine parts and suspension related parts of automobiles and motorcycles. In the induction quenching, metal (conductive element) is subjected to high-frequency induction heating to be quenched. In the induction quenching of the steel product, the effective hardened layer depth and the total hardened layer depth are specified for the depth of a hardened layer after surface quenching (hereinafter, referred to as hardness penetration) and the hardness of the hardened layer. Therefore, in order to guarantee the quality of a steel product, it is necessary to measure and evaluate the hardness penetration and the hardness.
In an existing art, a sampled steel product is partially cut, the cross-sectional strength is measured by various hardness testers, such as a Vickers hardness tester, and then the hardness penetration and the hardness are evaluated on the basis of the measured result. However, a sampled steel product is discarded in this method through the destructive inspection. This leads to an increase in material cost. In addition, a period of time required for inspection elongates, and 100 percent in-line inspection is not easy, so there is a possibility that a single isolated failure cannot be detected.
Then, there is known a technique for measuring the hardness penetration and hardness of a steel product through eddy current inspection that is nondestructive inspection (for example, see Japanese Patent Application Publication No. 2009-133694 (JP-A-2009-133694) and Japanese Patent Application Publication No. 2007-40865 (JP-A-2007-40865)). In the eddy current inspection, an excitation coil through which alternating-current flows is brought close to the steel product to generate an alternating-current magnetic field, the alternating-current magnetic field is used to generate eddy current in the steel product, and then an induction magnetic field induced by the eddy current is detected by a detection coil. That is, it is possible to quantitatively measure the hardness penetration and hardness of a steel product in a short period of time in 100 percent inspection by means of the eddy current inspection without discarding the steel product. The eddy current inspection is used in not only a hardness penetration and hardness measurement test (hereinafter, referred to as hardness penetration measurement test) for measuring the hardness penetration and hardness of a steel product but also a flaw detection test for detecting a flaw, such as a crack, that arises on the surface of an inspection object, a material discrimination test for detecting foreign matter contained in an inspection object, and the like.
The steel product has a difference in magnetic permeability between a base material and a martensite that arises in a hardened layer. Thus, when the steel product is measured using an eddy current sensor, the voltage (amplitude) detected by the detection coil varies with a variation in the hardness penetration. In addition, the voltage detected by the detection coil monotonously reduces with an increase in the depth of the hardened layer. In a hardness penetration measurement test, these phenomena are used to make it possible to calculate the hardness penetration of a steel product.
For example, in the technique described in JP-A-2009-133694, an encircling coil is used to detect the hardness penetration of a shaft portion of a shaft-like component. The encircling coil is stronger in magnetic field than a probe coil (hereinafter, simply referred to as “probe coil”) that has a probe formed of a coil of which the axial direction is oriented vertically with respect to a steel product to inspect the hardness penetration of the steel product, and a distance of the encircling coil to the steel product does not need to be precisely controlled, so the encircling coil is suitable for the hardness penetration measurement test. However, the inside diameter of the encircling coil, which is a measuring portion, is constant, so the filling rate of a measured portion with respect to the encircling coil (the rate of the cross-sectional area of the measured portion of the steel product with respect to the cross-sectional area of the inner circle of the encircling coil) varies with the outside diameter of the measured portion of the steel product. As the filling rate decreases, the accuracy of eddy current inspection exponentially decreases, so there is a problem in the related art that the outside diameter of the steel product varies among the measured portions and, as a result, a difference in inspection accuracy arises. In addition, the steel product that is the inspection object needs to be inserted in the encircling coil, so the application of eddy current inspection is limited to a shaft-like component having a substantially constant outside diameter. That is, it is difficult to set a component, such as a crankshaft, of which the outside diameter varies by a large amount, as an inspection object.
In addition, according to a technique described in an embodiment of JP-A-2007-40865, a probe coil is used to measure the hardness penetration of a steel product. The ratio of a detecting signal component with respect to a noise component is smaller in the hardness penetration measurement test than in another flaw detection test or a material discrimination test, so higher detection accuracy is required. However, the probe coil generates a weak magnetic field and needs to precisely control a distance to a steel product, so the probe coil may be applied to a flaw detection test or a material discrimination test but it is difficult to employ the probe coil for a hardness penetration measurement test.
In addition, when the probe coil is used to conduct hardness penetration measurement test over a steel product, a variation in output value due to a variation in the hardness penetration of the steel product and a variation in output value due to a variation in lift-off (a measuring distance of the probe coil to the steel product) are output with the same characteristic: Specifically; as shown in FIG. 16, even when the hardness penetration varies or even when the lift-off varies, variations in measured eddy current values X and Y, which are output values, appear on an X-Y coordinate plane. Therefore, the influence of the lift-off is significantly received in measuring the hardness penetration, so it is difficult to improve the measurement accuracy.
On the other hand, for example, Japanese Patent Application Publication No. 2009-69090 (JP-A-2009-69090) describes a technique for conducting eddy current inspection in such a manner that two excitation coils of which axes are oriented in a direction parallel to an inspection object surface of a steel product are arranged so as to be perpendicular to each other and a detection coil is provided at an intersecting portion of the excitation coils.
According to the technique described in JP-A-2009-69090, two excitation coils are combined so that the axes are perpendicular to each other, and the detection coil is arranged at the intersecting portion of the excitation coils. Then, alternating-current voltage is applied to the excitation coils in a state where the detection coil faces the steel product (that is, the axes of the two excitation coils are parallel to the inspection object surface of the steel product) to thereby conduct eddy current inspection.
However, the technique described in JP-A-2009-69090 is used for a flaw detection test through eddy current inspection, and it is difficult to apply the technique to a hardness penetration measurement test. That is, as described above, the ratio of a signal component with respect to a noise component is smaller in the hardness penetration measurement test than in the flaw detection test, so further higher detection accuracy is required. In addition, the flaw detection test may be determined on the basis of whether there is a flaw signal within a detection signal, whereas the hardness penetration measurement test needs to measure the magnitude of a detection signal. For these reasons, the hardness penetration measurement test more easily receives the influence of lift-off than the flaw detection test, so the configuration of a flaw detection test using eddy current inspection in the related art is not able to accurately conduct hardness penetration measurement test.